Periodic Reporting for period 1 - MUSICA (Origin of Magnetic strUctureS in the ISM obscuring the Cosmic DAwn)
Période du rapport: 2020-02-01 au 2022-01-31
Cosmic magnetism is among the biggest unknowns in the formation of structures in the Universe. Radio observations represent a unique window to explore magnetic fields from the most diffuse and tenuous intergalactic media to the turbulent interstellar medium (ISM) of the Milky Way. The ISM is the space that fills most of the Galaxy between already existing stars and planets. It constitutes the mass reservoir for the formation of new stars that partake to the life cycle of the Galaxy. The ISM is a dynamical system made of multiphase gas (with different thermodynamical equilibrium conditions), dust particles, cosmic rays, and magnetic fields. The complex interaction among its constituents regulates Galactic evolution as a whole determining the origin of cold-dense pockets of gas, called molecular clouds, where new solar systems are born. Thus, the study of structure formation in the ISM is intimately connected to societal fundamental questions about the origin of stars, planets, and life in the Universe and the place that human beings occupy in Nature. Moreover, a thorough understanding of the Galactic ISM represents a pivotal issue for accessing with high accuracy extragalactic and cosmological signals that unavoidably reach our telescopes through the magnetized ISM. The problems of structure formation in the ISM and that of Galactic foregrounds in the radio band were our two scientific pillars in the course of our MSCA-IF project, called MUSICA.
We aimed to focus on one specific aspect of structure formation in the ISM, namely the role that magnetic fields play in the formation of cold gas in the Galaxy. Our goal was to deepen the understanding of unprecedented radio observations of the magnetized Milky Way at low frequency (< 200 MHz) from the LOFAR telescope. These data offered a completely new perspective to observing the diffuse ISM toward the formation of cold gas in the Galaxy. The low-frequency radio sky is dominated by synchrotron radiation resulting from the interaction of cosmic-ray electrons gyrating around interstellar magnetic fields. However, at these low frequencies synchrotron radiation is also heavily affected by a wavelength-dependent process called Faraday rotation. The longer the wavelength the stronger the effect. Because of Faraday rotation, synchrotron polarization is rotated depending on the amount of ionized and magnetized gas along any given sightline.
Thanks to its broadband receivers and high sensitivity, the LOFAR telescope provided us with the finest-ever survey of the magneto-ionic medium of our Galaxy through Faraday rotation. We spotted regions in the sky where the polarization observed with LOFAR was strongly correlated with tracers of the cold neutral interstellar gas rather than with the fully ionized gas. This opened the unprecedented possibility of probing the mutual interaction of warm and cold gas phases in the diffuse ISM under the action of magnetic fields. Our overall objectives consisted in statistically quantifying the correlation between tracers of the multiphase and magnetized ISM based on multi-wavelength observational data and to start modelling the observed signals based on state-of-the-art numerical simulations of the magnetized Galaxy. Our goal was to identify the key physical processes that give origin to the sky brightness in polarization observed with LOFAR and to provide first a statistical description of the synchrotron sky at low frequency.
We aimed to focus on one specific aspect of structure formation in the ISM, namely the role that magnetic fields play in the formation of cold gas in the Galaxy. Our goal was to deepen the understanding of unprecedented radio observations of the magnetized Milky Way at low frequency (< 200 MHz) from the LOFAR telescope. These data offered a completely new perspective to observing the diffuse ISM toward the formation of cold gas in the Galaxy. The low-frequency radio sky is dominated by synchrotron radiation resulting from the interaction of cosmic-ray electrons gyrating around interstellar magnetic fields. However, at these low frequencies synchrotron radiation is also heavily affected by a wavelength-dependent process called Faraday rotation. The longer the wavelength the stronger the effect. Because of Faraday rotation, synchrotron polarization is rotated depending on the amount of ionized and magnetized gas along any given sightline.
Thanks to its broadband receivers and high sensitivity, the LOFAR telescope provided us with the finest-ever survey of the magneto-ionic medium of our Galaxy through Faraday rotation. We spotted regions in the sky where the polarization observed with LOFAR was strongly correlated with tracers of the cold neutral interstellar gas rather than with the fully ionized gas. This opened the unprecedented possibility of probing the mutual interaction of warm and cold gas phases in the diffuse ISM under the action of magnetic fields. Our overall objectives consisted in statistically quantifying the correlation between tracers of the multiphase and magnetized ISM based on multi-wavelength observational data and to start modelling the observed signals based on state-of-the-art numerical simulations of the magnetized Galaxy. Our goal was to identify the key physical processes that give origin to the sky brightness in polarization observed with LOFAR and to provide first a statistical description of the synchrotron sky at low frequency.
The MUSICA project accomplished two principal milestones that were foreseen in the proposal.
1) The first milestone was observational. We managed to combine LOFAR polarization data with tracers of the diffuse neutral interstellar medium (ISM), namely with the spectroscopic emission of atomic hydrogen (HI) at 21 cm. LOFAR polarization data below 200 MHz are a probe of diffuse synchrotron radiation in our Galaxy affected by Faraday rotation. While synchrotron emission should not depend on the gas phase, Faraday rotation should highlight regions along the line of sight of ionized and magnetized gas. However, we spotted regions in the sky where LOFAR data showed correlation with the neutral ISM.
We produced the first statistical analysis on the correlation of the two datasets (i.e. LOFAR and HI) using advanced tools, such as the histograms of oriented gradients (HOG, Soler et al. 2019). We managed to segment the multiphase gas probed by the HI emission using the ROHSA algorithm (Marchal et al. 2019). With the help of the secondment partner (M.-A. Miville Dechênes, CEA/Saclay), we presented the first statistical result on the LOFAR/HI correlation in Bracco et al. 2020 (A&A, 644, L3). We found a strong correlation between the polarized intensity seen by LOFAR and the cold phase (T < 100 K) of the HI gas toward a couple of fields of view at intermediate Galactic latitude.
2) The second milestone was the interpretation of our observational result on the correlation between tracers of the multiphase and magnetized ISM. We employed state-of-the-art magneto-hydrodynamic (MHD) numerical simulations of the multiphase gas in the Galaxy from Bellomi et al. 2020 and Ntormousi et al. 2017. We produced first synthetic observations of synchrotron emission that could be thoroughly compared to real data.
The first step forward in the understanding of low-frequency synchrotron emission was presented in Padovani, Bracco et al. 2021 (A&A, 651, A116), where we showed how considering accurate properties for the cosmic rays responsible for synchrotron radiation (i.e. a variable cosmic-ray energy spectrum) is crucial to interpret the amount of synchrotron emission observed at low frequency with LOFAR today, and with SKA in the future.
Building on this result, we produced the most realistic synthetic observations of synchrotron polarization data below 200 MHz presented in Bracco et al. submitted to A&A (11/2021). These simulations model the synchrotron polarization derived from multiphase and magnetized gas compressed by two large fronts, reminiscent of supernova remnants in the diffuse ISM. We produced the first complete analysis in simulations of the gas-phase contribution to synchrotron emission and Faraday rotation using standard algorithms like HOG. We found that simulations suggest that a strong amount of both Faraday rotation and synchrotron emission at low frequency may come from warm partially ionized gas and warm neutral gas, which could trace the formation of cold gas observable with HI emission.
1) The first milestone was observational. We managed to combine LOFAR polarization data with tracers of the diffuse neutral interstellar medium (ISM), namely with the spectroscopic emission of atomic hydrogen (HI) at 21 cm. LOFAR polarization data below 200 MHz are a probe of diffuse synchrotron radiation in our Galaxy affected by Faraday rotation. While synchrotron emission should not depend on the gas phase, Faraday rotation should highlight regions along the line of sight of ionized and magnetized gas. However, we spotted regions in the sky where LOFAR data showed correlation with the neutral ISM.
We produced the first statistical analysis on the correlation of the two datasets (i.e. LOFAR and HI) using advanced tools, such as the histograms of oriented gradients (HOG, Soler et al. 2019). We managed to segment the multiphase gas probed by the HI emission using the ROHSA algorithm (Marchal et al. 2019). With the help of the secondment partner (M.-A. Miville Dechênes, CEA/Saclay), we presented the first statistical result on the LOFAR/HI correlation in Bracco et al. 2020 (A&A, 644, L3). We found a strong correlation between the polarized intensity seen by LOFAR and the cold phase (T < 100 K) of the HI gas toward a couple of fields of view at intermediate Galactic latitude.
2) The second milestone was the interpretation of our observational result on the correlation between tracers of the multiphase and magnetized ISM. We employed state-of-the-art magneto-hydrodynamic (MHD) numerical simulations of the multiphase gas in the Galaxy from Bellomi et al. 2020 and Ntormousi et al. 2017. We produced first synthetic observations of synchrotron emission that could be thoroughly compared to real data.
The first step forward in the understanding of low-frequency synchrotron emission was presented in Padovani, Bracco et al. 2021 (A&A, 651, A116), where we showed how considering accurate properties for the cosmic rays responsible for synchrotron radiation (i.e. a variable cosmic-ray energy spectrum) is crucial to interpret the amount of synchrotron emission observed at low frequency with LOFAR today, and with SKA in the future.
Building on this result, we produced the most realistic synthetic observations of synchrotron polarization data below 200 MHz presented in Bracco et al. submitted to A&A (11/2021). These simulations model the synchrotron polarization derived from multiphase and magnetized gas compressed by two large fronts, reminiscent of supernova remnants in the diffuse ISM. We produced the first complete analysis in simulations of the gas-phase contribution to synchrotron emission and Faraday rotation using standard algorithms like HOG. We found that simulations suggest that a strong amount of both Faraday rotation and synchrotron emission at low frequency may come from warm partially ionized gas and warm neutral gas, which could trace the formation of cold gas observable with HI emission.
Our results opened new research pathways that we are exploring observationally and numerically. We are investigating how to trace the warm partially ionized gas in real data and how to model it more carefully using non-analytical derivation of the ionization state of the ISM. Our work is stressing the importance of considering the dynamical evolution of interstellar gas across different phases in the process of cold-gas formation in the turbulent and magnetized Galaxy. Our modeling will be a stepping stone for characterizing diffuse Galactic signals also in the context of Galactic foregrounds for the detection of the 21-cm signal from cosmic dawn and the Epoch of Reionization. Our work will become a reference to produce forecasts for future low-frequency telescopes, from LOFAR 2.0 to NenuFAR, and the SKA.